Superconducting wire for conducting high-intensity currents



Jan. 23, 1968 H. VOlGT 3,365,538

SUPERCONDUCTING WIRE FOR CONDUCTING HIGH-INTENSITY CURRENTS Filed April 6, 1965 HARD SUPERCONDUCTORS HARD SUPERCONDUCTORS Flg 1o Fig.1b

n m 211 an 1.0 5'0 H [E] 3,365,538 SUPERCUNDUCTING WIRE FOR QGNDUCTING HIGH-INTENSITY CURRENTS Hans Voigt, Erlangen, Germany, assignor to Siemens- Schucirertwerke Alrtiengesellschait, Berlin-Siemensstadt, Germany, a corporation of Germany Filed Apr. 6, 1965, Ser. No. 446,007 Claims priority, application Germany, Apr. 17, 1964, S 90,596 6 Claims. (Ql. 174-128) My invention relates to superconducting wires or acbles, particularly for conducting currents of high intensity.

Superconductors can carry electric currents up to a given critical current intensity only. If this limit is exceeded, the superconductor converts to normal conductance. The critical current intensity depends upon various conditions, including the particular material, the external shape of the particular superconducting structure, the temperature and the magnetic fields acting upon the superconductor.

Published measuring results (-Sekula, Boom and Bergeron, Appl. Phys. Letters 2; 1963; page 102) have shown that cold-drawn wires of different hard superconducting materials when subjected to longitudinal magnetic fields these are magnetic fields in the direction of the current fioWexhi-bit considerably higher critical current limits than without the magnetic field. This effect has been ascertained for example with Wires of niobium-zirconium alloys of different compositions and With wires of molybdenum-rhenium (33% Rhe) and tantalum-titanium (25% Ti).

It is an object of my invention to provide a wire or cable structure of superconducting material which is capable of carrying more electrical current by virtue of a higher critical current intensity, without the necessity of employing any separate or external means for producing a longitudinal magnetic field.

To this end, and in accordance with a feature of my invention, I design the superconducting wire structure of hard superconducting material in such a manner that the current flowing through the structure generates by itself a longitudinal magnetic field, thus raising the critical limit of the current intensity above which the material converts to normal conductance.

In accordance with another, more specific feature of my invention, the wire-shaped or cable-shaped superconductor structure is provided with a number of current paths which extend along and substantially helically about the conductor axis, and which are structurally distinct from one another and joined together to form a single conductor Wire or cable structure. The wire structure may also contain one or more current paths which extend in the axial region of the conductor in a direction parallel to the geometric axis of the wire structure.

According to another feature of my invention, a particularly favorable design of the Wire or cable-shaped superconductor structure is obtained by having the pitch of the helical current paths decrease from the conductor axis toward the surface of the conductor. As a result, the current paths in the vicinity of the conductor axis extend essentially in the axial direction, whereas the current paths closer to the conductor surface follow a more circular course about the conductor axis.

A qualitative consideration shows that a current configuration which possesses in the vicinity of the axis a slight azimuthal component and increases with increasing radial distance from the axis, causes a similar magnetic field configuration. In a greatly simplified manner, this can be imagined to be tantamount to the fact that the outer current paths extending approximately circularly are located in the circular magnetic field of the inner current paths extending approximately in the axial direction, whereas the inner current paths of substantially axial direction are subjected to the magnetic field of the outer, approximately circular current paths. Electric current and magnetic field are then approximately parallel at each locality in the interior of the superconductor structure. In other Words, the current flowing in the superconductor structure generates for itself the required longitudinal field.

The critical current intensity of a superconductor structure thus formed according to the invention and exhibiting substantial parallelism of current and magnetic fields, is increased in a manner similar to the increase in critical current intensity observed in a normal coldrolled superconducting wire when exposed to a longitudinal magnetic field externally applied.

Suitable as material for superconductor structures according to the invention are all of the superconducting materials that exhibit an increase in critical current intensity within a longitudinal magnetic field. This applies particularly to the superconducting alloys niobium-zirconium, molybdenum-rheniurn and tanta'lum-titanium.

A particularly large extent of parallelism between current and magnetic field in a superconducting structure according to the invention is attained if those current paths that are spaced from the conductor axis a distance equal to one-half the conductor radius, extend on such a course that the azimuthal and longitudinal components of the currents flowing through these particular paths are approximately equal. This is the case if the pitch height, that is the axial length of an individual helical turn, of the helical current paths, is of the same order of magnitude as the circumference of the geometrical cylinder defined by these current paths.

However, this feature as well as the above-mentioned preferred feature of having a helical pitch which decreases from the conductor axis toward its periphery, need not necessarily be embodied in the superconductor structure in order to achieve an appreciable increase in critical current intensity. This is because with a helical course of the current paths there occur in any event longitudinal magnetic field components with the eifect of raising the critical current limit.

According to still another, preferred feature of my invention, a superconducting Wire structure is composed of a bunch of thin superconducting individual wires helically extending about the longitudinal axis of the bunch. A superconducting wire may also be arranged on the axis of the bunch so as to extend parallel to the axis. Instead of a single central wire, several superconducting wires may be located in the axial center region of the structure and extend parallel to the axis.

The invention will be further described with reference to an embodiment illustrated by way of example on the accompanying drawing in which:

FIG. 1a is a front view of a superconducting wire structure according to the invention.

FIG. 1b is a lateral view of the same structure in three portions showing the inner axial wire, an intermediate group of wires, and the outer wires respectively; and

FIG. 2 is a graph of measuring results obtained with a wire structure according to the invention.

The cable-type superconductor structure shown on enlarged scale in FIGS. 1a and 1b is composed of seven superconducting wires of the same diameter consisting of niobium-Zirconium alloy or one of the other hard superconducting materials having a magnetically responsive critical current limit. A center Wire 1 extends parallel to the conductor axis. Four superconducting wires 2 are helically Wound upon the center wire l with such a helical pitch that the axial height of each winding turn corresponds approximately to the circumference of the cylinder defined by the helix. This helix, therefore, has a pitch of approximately 45 so that the azimuthal and longitudinal components of the current passing through the helical wires are approximately equal. A third layer of the cable structure consists of two superconducting wires 3 which are Wound at a smaller pitch helically upon the layer formed by the four wires 2. The current flowing through the wires 3 produces a magnetic field approximately parallel to the cable axis.

Cable-type superconductor structures according to the invention may be composed of numbers of wires and winding layers diiierent from those of the illustrated embodiment. They may also be composed of wires having respectively different diameters.

Another way of producing a superconductor structure according to the invention is to employ a single wire which is torsionally deformed about its own longitudinal axis. Such a wire is produced, for example, by subjecting a cold-rolled wire of superconducting alloy to torsion. During such treatment, the dislocation lines, originally extending in the longitudinal direction of the wire and determining the direction of the current-flow paths, become helically twisted about the wire axis. By thus torsionally stressing the material beyond the limit of elastic deforma tion, the deformation of the dislocation lines to helical shapes becomes permanently embodied in the structure and thus forms a structurally distinct arrangement of helical current paths. For that reason, the twisted and plastically deformed wire is capable of conducting higher currents than an otherwise identical but not torsionally twisted wire without converting from superconductance to normal conductance. As a rule, it sufiices to twist the wire at the approximate rate of one full turn for an axial length equal to about four times the wire diameter.

This will be apparent from the following example.

A cold-rolled wire of a niobium-% zirconium alloy having a diameter of 0.254 mm. (Manufacturer: Wah Chang) was twisted about its own axis one full turn per millimeter length. Upon cessation of the twisting torque, the deformation declined elastically by 5% so that a plastic, permanent deformation of 0.95 turn per millimeter length of wire remained.

The critical current limit of a piece of this wire, 27 cm. long, was measured first without external field and thereafter in magnetic fields oriented perpendicularly to the wire axis. For comparison, the critical current intensity was measured of an identical but not torsionally deformed wire. All of the measurements were made at a temperature of 4.2 K. The measuring results are represented in the diagram of FIG. 2 indicating the magnetic field H in kilogauss (kg) along the abscissa and the current intensity I in amps (a.) on the ordinate. Curve a indicates the critical current intensities of the nontwisted wire in dependence upon a transversal magnetic field. The critical current intensity is approximately 150' a. in fieldfree space and decreases with increasing magnetic field down to about 30 a.

Curve b shows the critical current intensity of the torsionally deformed wire according to the invention. Without magnetic field, this critical current intensity is about 225 a., and it decreases with increasing field down to about 45 a. The increase in critical current limit achieved by twisting of the wire is clearly apparent from the two curves in FIG. 2.

It is desirable to subject the wire of hard superconducting material to twisting deformation while the Wire is being produced by drawing it through a die, thus combining the torsional deformation with the drawing operation.

Undesired disturbances in the texture of the wire, as may occur with the twisting operation, can be cured by subsequent heat treatment of the Wire. For example, niobium-zirconium "wires, after being twisted to permanent torsional deformation. in the above-described manner, are

4 heated for about 30 minutes to 5 hours at 600 to 800 C. For wires of small diameter, according to the foregoing example, a heat treatment at about 800 C. for 2 hours is preferable. The heat treatment is effected under protective gas. Curve 1) in FIG. 2 was obtained with a twisted wire not subjected to heat treatment.

Superconducting wire structures according to the invention are applicable for example as cables for transporting electric high-intensity currents. Superconducting wires according to the invention are also well suitable for winding the outer layers of superconducting magnet coils in which the occurring transversal magnetic fields are still relatively weak.

I claim:

1. A superconducting wire structure of hard superconducting material comprising a bunch of Wires formed of hard superconducting material, at least one of said wires extending substantially along the axis of the wire structure, said other wires extending along and helically about said one wire and joined together to form a single conductor, the inner wires having a steeper helical pitch than the paths closer to the conductor surface.

2. A superconducting wire structure according to claim 1, comprising among said helical Wires a group spaced from the axis a distance equal to one-half of the conductor radius, said latter helical wires having an axial height of the individual turns in the same order of magnitude as the circumference of the cylinder defined by said group of helical wires.

3. A superconducting wire structure according to claim 1, wherein the wires consist of a single piece having a torsionally deformed internal texture forming helical current paths.

4. A superconducting wire structure comprising a bunch of individual wires of niobium-zirconium alloy, one of said wires extending along the axis of said wire structure, a first group of Wires helically wound about said axis and a second group of wires helically wound about said first group, the first group having a steeper helical pitch than said second group.

5. A superconducting wire structure comprising a bunch of individual wires of molybdenum-rhenium alloy, one of said Wires extending along the axis of said wire structure, a first group of wires helically Wound about said axis and a second group of wires helically wound about said first group, the first group having a steeper helical pitch than said second group.

6. A superconducting wire structure comprising a bunch of individual wires of tantalum-titanium alloy, one of said wires extending along the axis of said wire structure, a first group of wires helically Wound about said axis and a second group of wires helically wound about said first group, the first group having a steeper helical pitch than said second group.

References Cited UNITED STATES PATENTS 1,999,273 4/1935 Austin 174128 3,215,905 11/1965 Bloom 336-208 X 112,137 2/1871 Fricke 174-428 626,940 6/1899 Smith l74128 2,946,030 7/1960 Slade 3l7-158-1 3,185,900 5/1965 Jaccarino 317158.1

FOREIGN PATENTS 952,226 3/ 1964 Great Britain.

OTHER REFERENCES Solenoid Magnet ls Superconductive, Chemical Eng. News, Feb. 20, 1961, p. 4l.

LEW ES H. MYERS, Primary Extmzirter.

E. GOLDBERG, Assistant Examiner. 

1. A SUPERCONDUCTING WIRE STRUCTURE OF HARD SUPERCONDUCTING MATERIAL COMPRISING A BUNCH OF WIRES FORMED OF HARD SUPERCONDUCTING MATERIAL, AT LEAST ONE OF SAID WIRES EXTENDING SUBSTANTIALLY ALONG THE AXIS OF THE WIRE STRUCTURE, SAID OTHER WIRES EXTENDING ALONG AND HELICALLY ABOUT 